Pressure or force measuring device

ABSTRACT

A force or pressure sensor has a measuring body exposed to the measuring force and a reference body, said bodies both being supported at the housing via force measuring elements. In an evaluating circuit the signals of the force measuring elements, the second time derivatives of said signals and possibly also their first time derivatives are linked in such a manner that a signal representing the measuring force is obtained which is largely independent of the dynamic inherent behavior of the pressure sensor on shocks and vibrations of the housing and on rapid changes of the measuring force.

DESCRIPTION

The invention relates to a pressure or force measuring device comprisinga housing, a measuring body subjected to a measured pressure or measuredforce, signal generators for generating measuring signals correspondingto the deformation of the body, and an evaluating circuit processing themeasuring signal for generating an output signal proportionate to themeasuring pressure or measuring force.

When measuring pressures which vary with time, measuring errors occurdue to mass accelerations in the mechanical part of such pressuresensors. Pressure shocks, stochastic pressure variations and externaldisturbances, for example vibrations acting on the housing, leadrepeatedly to transient or settling phenomena. During this transientphenomena the measured values indicated have dynamic errors.Fundamentally, due to its construction consisting of the mass of thepressure-influenced part and the return force of the force measuringelement acting as spring, the pressure sensor reacts with a dynamicbehaviour corresponding to a damped spring-mass system.

DE-PS 694,803 discloses a pressure sensor of the type mentioned at thebeginning in which the measuring body subjected to the measured pressureand a reference body are each supported via piezoelectric measuringelements on the housing, the measuring signals of the two force metersbeing superimposed subtractively on each other. If during a pressuremeasurement vibration forces act on the housing they are transmitted viathe two force measuring elements to the measuring body and referencebody and generate equisized and equidirectional signals which canceleach other out so that only the pressure force exerted by the measuredpressure on the measuring body is measured. However, fundamentally withthis arrangement only the acceleration forces acting on the housing canbe excluded from the measurement. Dynamic measuring errors due to suddenchanges in the pressure to be measured are not covered. However, eventhe dynamic components of the forces acting on the housing arecompletely compensated only if in an ideal case an exactly synchronousmovement of measuring body and reference body is assumed on housingvibrations or shocks. Because of unequal masses, material damping, etc.,in a real case phase displacements between measuring and referencebodies necessarily occur and falsify the measurement result.

The invention is based on the problem of further developing a pressureor force sensor of the type indicated with the simplest and mosteconomical construction possible in such a manner that a measurementresult is obtained which as far as possible is not falsified by thedynamic inherent behaviour of the sensor, in particular on rapidlychanging measured pressures or forces.

By the invention, the force components generated by the dynamicspring-mass behaviour of the sensor are detected and compensated in theevaluating circuit so that the measuring signal obtained representssubstantially exactly the time profile of the force to be measured. Thisis done in that by the evaluating means at any instant the dynamic forceequilibrium for both masses is calculated at least in simpleapproximation, and indeed exactly in accordance with the furtherdevelopment.

A further advantage of the invention resides in that the force orpressure sensor can be made substantially without damping. Because oftheir dynamic behaviour, hitherto usual pressure or force sensorsrequire a damping to minimize as far as possible the influence of thetransient or settling phenomena on the measurement result. However, sucha damping in the sensor system also suppresses the force peaks of theforce profile to be measured and lead in particular with high-frequencypressure or force variations to erroneous measurement. The inventionpermits a practically damping-free construction of the sensor so thatthe latter can respond substantially without delay to changes in forceor pressure.

Embodiments of the invention will be explained in detail with the aid ofthe drawings, wherein:

FIG. 1 is a schematic section through a first embodiment of a pressureor force sensor according to the invention;

FIG. 2 is a simplified basic sketch of the sensor of FIG. 1 toillustrate the forces occurring on deflections;

FIG. 3 is the basic scheme of an evaluation for the sensor of FIG. 1;

FIG. 4 is a detail of a modified embodiment of the sensor of FIG. 1;

FIG. 5 is the scheme of an evaluating circuit for the embodimentaccording to FIG. 4;

FIG. 6 shows in an illustration similar to FIG. 1 a section through amodified embodiment of the pressure or force sensor.

FIG. 7a is a separate illustration of the components of the sensor ofFIG. 6 and

FIG. 7b is the associated basic sketch for deriving the force equations;

FIG. 8 is a sketch for taking account of the damping forces;

FIG. 9 shows the scheme of an evaluating circuit for the embodiment ofFIG. 6;

FIGS. 10 and 11 show two modified embodiments of the pressure sensoraccording to FIG. 6.

According to FIG. 1, in a for example cylindrical housing 1 a measuringbody 3 is supported by means of a for example tubular firstspring-elastic force element 5. Pressure to be measured acts on the endface of the measuring body 3, which is sealed with respect to thehousing 1 by means of a diaphragm 7, and exerts a pressure force F_(m).If the sensor is to be used as force sensor the force F_(m) to bemeasured can be transmitted by means of suitable elements to themeasuring body 3. The length change of the force measuring element 5caused by the force F_(m) can be detected by means of strain gauges 9.

In the interior of the housing 1 a reference body 11 is arranged whichis not exposed to the force F_(m) to be measured. In the embodimentshown it is supported by a second elastic deformable force measuringelement 13 with respect to the measuring body 3 and a signalproportional to the change of length of the force measuring element 13can be tapped off by means of strain gauges 15. Electrical connectionsof the strain gauges 9, 15 (not shown) can be led out through a bore 17in the housing 1.

Instead of constructing the force measuring elements 5, 13 as springelements with strain gauges in the manner illustrated, as known per sethey may also be made in the form for example of piezoelectric elements,inductive force pickups and the like.

In FIG. 2 the forces acting on the housing 1, measuring body 3 andreference body 11 are represented schematically. If the forces measuredin the force measuring elements 5 and 13 are denoted by F₁ and F₂respectively the masses of the measuring body 3 and reference body 11 bym and m_(r) respectively and the deflections of the housing 1, measuringbody 3 and reference body 11 by x₀, x₁ and x₂ respectively, as indicatedin FIG. 1, the following relationships are obtained:

Dynamic equilibrium at the measuring body 3:

    F.sub.m -F.sub.1 -F.sub.2 =m x.sub.1                       (1)

Dynamic equilibrium at the reference body 11:

    F.sub.2 =m.sub.R x.sub.2                                   (2)

Change of length of the force measuring element 13 between measuring andreference bodies:

    F.sub.2 =c.sub.2 (x.sub.1 -x.sub.2)                        (3)

wherein c₂ is the spring constant of the force measuring element 13.

By twice differentiating equation (3) and combining with (1) and (2) weobtain: ##EQU1## the last term of which is a correction term for allmeasuring errors caused by the dynamic inherent behaviour of the sensor.

FIG. 3 shows an example of embodiment for an evaluating circuit withwhich the signals F₁ and F₂ tapped off at the first and second forcemeasuring elements 5 and 13 respectively can be processed according tothe formula (4) to give the measuring signal corresponding to the forceF_(m) applied. The signal F₁ and the signal F₂ are supplied directly toa summation member 17. Furthermore, the signal F₂ is multiplied in amultiply member 19 by the quotient of the masses m and m_(R) of themeasuring body and reference body and supplied to the summation member17. Finally, the signal F₂ is differentiated twice in twodifferentiating members 21, 23 and in a multiplication member 25multiplied by the quotient of the mass m of the measuring body and thespring constant c₂ of the second force measuring element 13 and finallysupplied to the summation member 17. By summation of the input valuesthe summation member 17 forms an output signal which is proportional tothe measured force FM and can be displayed in a display member 29 and/orrecorded.

The various computing members of the evaluating circuit need not beformed as separate analog circuit elements as illustrated, it beingpossible of course to carry out a corresponding digital signalprocessing of the signals F₁ and F₂ corresponding to the formula (4) ina microprocessor or similar computer with previous analog-digitalconversion of the signals.

In the embodiment described with the aid of FIGS. 1 to 3 the influencesof the masses m and m_(R) of the measuring body and reference body aretaken into account. Depending on the design of the sensor and thefrequency range to be detected, however, the inherent masses of theforce measuring elements 9, 13 can also influence the dynamic behaviour.In this case, in the force balance account must also be taken of theforces originating from the accelerations of the force measuringelements 9, 13. This applies in particular to the force measuringelement 13 transmitting the forces between the measuring body 3 andreference body 11.

In FIG. 4 a detail of a modified embodiment of the sensor according toFIG. 1 is shown in which with additional strain gauges 14, 16 the forcesF₂ and F₃ transmitted by the force measuring element 13 to the measuringbody 3 and the reference body 11 respectively can be measuredseparately. The strain gauge 15 extends over the entire spring length ofthe force measuring element 13 and detects the force F_(2M)corresponding to the relative movement of the measuring body 3 andreference body 11. The inequality of the forces F₂ and F₃ is due to themass inertia of the force measuring element 13 on acceleration.

For the embodiment of FIG. 4 the aforementioned equations (1) to (4) areto be amended as follows: ##EQU2##

FIG. 5 shows the scheme of an evaluating circuit which links the forcesignals F₁, F₂, F₃ and F_(2M) according to equation (4') and also takesaccount of the material damping, inevitable in the system, in the springbodies. It is assumed here that a corresponding damping force D₁, D₂, D₃must be added to each of the respective forces F₁, F₂, F₃, each dampingforce depending in accordance with the formula

    D.sub.i =k.sub.i ·x.sub.i                         (5)

on the respective material damping constant Ki and the differentialx_(i) with respect to time of the deflection x_(i), the latter in turnin accordance with the formula ##EQU3## being equal to the quotient ofthe force F_(i) and the spring constant c_(i). If the axial lengthl_(DMS) of the respective strain gauge is smaller than the length l_(k)of the associated force measuring element (spring body), the localdetection detected by the strain gauge is ##EQU4## and the local dampingforce is obtained from (6a) and (5) as follows: ##EQU5## Putting##EQU6## then instead of the formula (4')the formula extended by thedamping forces is obtained: ##EQU7## In the evaluating circuit of FIG. 5the damping forces are taken into account by additional circuit branches30, 31, 32 with differentiating members 33, 34, 35 and multipliermembers 37, 38, 39 as well as summation members 41, 42, 43.

If for example when using very low-damping force measuring elementsthere is no need to take account of the damping forces, by omitting thecircuit branches 30, 31, 32 from FIG. 5 a simplified evaluating circuitis obtained for the embodiment according to FIG. 4.

FIG. 6 shows a modified embodiment of the pressure or force measuringdevice. In this embodiment the measuring body 65 on which the pressureforce F acts is supported via the force measuring element 63 withrespect to the reference body 66 and the latter in turn via the secondforce measuring element 64 with respect to the housing 61. The forcemeasuring elements 63, 64 here are not tubular but are made from solidmaterial; they can however expediently be made from a material differentto the measuring body 65 and reference body 66. The deformations of theforce measuring elements 63, 64 which occur on relative movements ofmeasuring body 65 and reference body 66 with respect to each other andwith respect to the housing 61 and are proportional to the forces thenoccurring may be detected by means of strain gauges 67, 68, 69, 70. Thesignal lines originating from the latter (not illustrated) can be ledout of the housing 61 through a bore 71.

In FIG. 7a the components of the pressure sensor of FIG. 6 areillustrated separately. The deformation forces F₁, F₂, F₂, acting at theinterfaces between the individual components, the damping forces D1, D2,D3 and the corresponding deflection coordinates X1, X2 for the masscentres S₁, S₂ of gravity and X0 for the housing are indicated. Themasses of the measuring body 65 and reference body 66 will be denoted byM1 and M2. Application of the mass centre of gravity law of mechanicsthen gives the following relationships:

Dynamic equilibrium for mass M1 and M2:

    F-F1-D1=M1·X1                                     (11)

    F2+D2-F3-D3=M2·X2                                 (12)

Kinematic relationship between the masses M1 and M2:

    X1=X2+Xrel                                                 (13a)

    X1=X2+Xrel                                                 (13)

The equation (13) is generally known as "absolute acceleration=guideacceleration+relative acceleration", applied to the movement between themasses M1 and M2.

If after appropriate transformation the terms (13) and (12) are insertedinto the term (11), we have:

    F=F1+D1+(M1/M2)·(F2+D2-F3-D3)+M1·Xrel    (14)

the last term of which represents a correction term for all themeasuring errors caused by the dynamic inherent behaviour of the sensor.

According to FIG. 7a, the forces F1, F2 and F3 are detected by thestrain gauges 68, 69 and 70. The gauge strips should be as short aspossible to ensure an exact determination, based on the derivative, ofthe forces acting in the transition cross-sections of the spring bodies63, 64 and the masses 65, 66.

The (inner) material damping forces D1, D2 and D3 are derived in theforce measuring elements in accordance with FIG. 8. The force F1 acts onthe section illustrated of the force measuring element 63 of length L,which is equal to the length of the strain gauge 68, and generates thechange of length

    ΔL=F1/C1                                             (15)

wherein C1 is the spring constant of the length section L. Thedeformation gives rise to the damping force D1 which is proportional tothe deformation rate: ##EQU8## wherein K1 is a material dampingconstant. It follows from (15) and (16) that ##EQU9## wherein A1=K1/C1is a constant. Equation (17) applies analogously to the damping forcesD2 and D3 at the location of the strain gauges 69 and 70.

The correction term M₁ ·X_(rel) of equation (14), which takes account ofthe acceleration of the mass centres S1, S2 of gravity relatively toeach other, can be derived from the measuring signal of the strain gauge67 as follows. Said output signal can be put equal to a force F_(M)which in the force measuring element 63 of length L_(M) results in thechange in length ΔL_(M) :

    F.sub.M =ΔL.sub.M ·C.sub.M                  (18)

wherein C_(M) is the spring constant of the force measuring element overthe length L_(M) thereof. Neglecting any deformations in the measuringbody 65 and reference body 66 itself, ΔL_(M) =X_(rel), and thus

    X.sub.rel =F.sub.M /C.sub.M                                (19)

By inserting (17) and (19) in (14) we have:

    F=F1+A1·F1+(M1/M2)·(F2+A2F2-F3-A3·F3)+(M1/C1).multidot.FM                                                  (20)

wherein F1, F2, F3 and F_(M) are the measuring signals of the straingauges 67, 68, 69, 70 and A1, A2, A3, M1, M2 and C1 are constants.

This equation (20) describes the movement of the masses M1 and M2 underthe action of the force F(t). The force F(t) acting at the instant t iscalculated from the motion of the masses.

FIG. 9 shows an example of embodiment for an evaluating circuit withwhich the tapped-off signals F1, F2, F3 and FM can be processedcorresponding to the formula (20). The signal F1 is supplied directly toa summation member 72, the signal F2 is supplied thereto aftermultiplication by the ratio of the two masses M1 and M2 in themultiplier 73 and the signal F3 is supplied thereto after sign reversalin the inverter 75 and multiplication in 73. The damping forces aredetermined by the circuit branches with differentiating members 78,multiplier members 75, 76, 77, D2 and D3 and additionally withmultiplication in 73 and D3 inverted in 74, and supplied to the adder72. The relative acceleration is calculated from the signal FM bydifferentiation twice in 78 and subsequent multiplication in 79 andadded in the adder 72 to the other terms of the equation (20). Thesummation of the input values in the adder gives a signal proportionalto the force F and is displayed in the display member 80.

In this case as well, instead of the evaluating circuit with separateanalog circuit elements, a corresponding digital signal processing ispossible in accordance with formula (20) in a microprocessor or similarcomputer, with previous analog-digital conversion of the signals.

In this case as well, if there is no need to take account of the dampingforces, the evaluation can be simplified by omitting the circuit andcomputing branches with the members 75, 76, 77.

The above derivation of equation (20) is based on the simplifiedassumption that a resilient deformation occurs only in the forcemeasuring elements 63, 64 and not in the masses 65, 66. In reality, bydeformations within the measuring body 65 and the reference body 66additional displacements of the centres S1 and S2 of gravity thereof canoccur which are not detected by the strain gauge 67. With extrememeasuring accuracy this can lead to erroneous detection of the relativemovements.

To minimize this error, constructional steps should be taken to ensurethat the deformations within the measuring body 65 and reference body 66are small compared with the deformations in the force measuring elements63 and 64. In the embodiment according to FIG. 6 this can be done bysuitable choice of materials with very different elasticity modulusesfor the force measuring elements 63, 64 on the one hand and themeasuring body 65 and reference body 66 on the other hand.

In accordance with FIGS. 10 and 11, however, the measuring body 65 andreference body 66 may also be integrally formed with the force measuringelements 63, 64 and consist of the same material as the latter. Inaccordance with FIG. 10 this then simply gives different portions of auniform cylindrical body whilst in accordance with FIG. 11 the forcemeasuring elements 63, 64 have a higher deformability due to athin-walled tubular configuration. In both cases it is advantageous forthe strain gauge 67 detecting the relative movement between themeasuring and reference body to have a length L5 which is greater thanthe length L2 of the force measuring element 63 and corresponds to thedistance between the centres S1 and S2 of gravity of the measuring andreference bodies.

The embodiments of FIGS. 6, 10 and 11 have basically only a singlerod-like body exposed to the measuring force F and supported against thehousing. Said body is divided over its length into various portions,i.e. the first mass portion or measuring body 65 on which the force Facts, a first deformation portion or force measuring element 63, asecond mass portion or reference body 66 and a second deformationportion or force measuring element 64. Signal generators in the form ofelongation measuring strips or strain gauges are attached to thedeformation portions 63, 64 at least at the points where the latteradjoin the mass portions 65, 66. It is not absolutely essential for thedeformation portions 63, 64 to differ from the mass portions 65, 66 intheir deformability. Consequently, the entire body may consist inaccordance with the embodiment of FIG. 10 of a uniform material ofconstant cross-section, the borders between the mass portions 75, 76 andthe deformation portions 63, 64 being defined only by the position ofthe strain gauges.

However, it is more advantageous for the deformation portions 63, 64 tohave a higher deformability than the mass portions 65, 66. For thispurpose, in accordance with FIG. 6 the entire rod-like body may be madeup of portions of different material with different elasticityproperties. Alternatively, it is also possible for the entire rod-likebody to consist integrally of the same material which is however treatedsectionwise so that it has different deformation properties. Forexample, the embodiment may consist of plastic having different Shorehardness in sections. The greater deformability of the deformationportions 63, 64 may then also be achieved by appropriate shaping so thatsaid portions have a smaller cross-section than the mass portions 65,66, for example by the tubular configuration shown in FIG. 11. Saidsteps of material choice and configuration may also be combined witheach other.

To make the measuring device as insensitive as possible to lateraldeformation or accelerating forces, in the embodiment according to FIG.6 for example the reference body or second mass portion 66 may besupported laterally with respect to the housing 61 by a diaphragm, thelatter being easily deformable in the direction of the measuring force Fbut as stiff as possible in the direction perpendicular thereto.

I claim:
 1. Pressure of force measuring device comprising a housing (1),a measuring body (3) subjected to the measured pressure or measured force, a reference body (11) not subjected directly to the measured pressure or measured force, a first elastically deformable force measuring element (13) via which the measuring body (3) and the reference body (11) are supported with respect to each other, a first signal generator (15) for generating a measuring signal corresponding to the deformation of the first force measuring element (13), a second elastically deformable force measuring element (5) via which the measuring body or the reference body is supported deflectably with respect to the housing, a second signal generator (9) for generating a measuring signal corresponding to the deformation of the second force measuring element (5), and an evaluating circuit processing the measuring signals from the two force measuring elements (5, 13), characterized in that the evaluating circuit comprises computing members (21, 23, 17) for forming the second time differential of the measuring signal from the first signal generator (15) and for additively combining of said second time differential with the undifferentiated measuring signals of the first and second signal generators (9, 15).
 2. Device according to claim 1, characterized in that the evaluating circuit additionally comprises computing members (31, 33, 35) for forming the first time differential of the measuring signals to take account of the damping behaviour of the device.
 3. Device according to claim 1, characterized in that at least on one of the force measuring elements (13) two signal generators (14, 16) are arranged in the vicinity of opposite ends of said one force measuring element (13).
 4. Measuring device according to claim 3, characterized in that in addition a signal generator or pickup (15) extending over the entire length said one of the force measuring element (13) is provided.
 5. Device according to claim 1, characterized in that the measuring body is supported by the first force measuring element with respect to the reference body and the latter is supported by the second force measuring element with respect to the housing.
 6. Measuring device according to claim 5, characterized in that the measuring body, reference body and force measuring elements are made integrally from the same material.
 7. Device according to claim 1, characterized in that the force measuring elements, by different material or different configuration, have a higher deformability than the measuring body and the reference body.
 8. Pressure or force measuring device comprising a rodlike or block-like body, the one end face of which is subjected to the measuring pressure or the measuring force and the other end face of which is supported with respect to a housing, signal generators for generating measuring signals corresponding to the deformation of the body, and an evaluating circuit processing the measuring signals for generating an output signal proportional to the measuring pressure or measuring force,characterized in that the body comprises the following portions over its length: a first mass portion (65) of predetermined mass and subjected to the measuring pressure or measuring force (F), a first elastically deformable deformation portion (63), a second mass portion (66) of predetermined mass, and a second elastically deformable deformation portion (64), that on the first and second deformation portions (63, 64) in each case at least one signal generator (67, 68, 69, 70) formed as a strain gauge is arranged and that the evaluating circuit comprises computing members (21, 23, 17) for forming the second time differential of the measuring signals of said one signal generator (67) of the first deformation portion (63) and for additively combining of said second time differential with the undifferentiated measuring signals of the signal generators of the two deformation portions (63, 64).
 9. Apparatus according to claim 8, characterized in that at least one said signal generator extends only over a small portion of the length of each deformation portion (63, 64) and is arranged at the boundary between the respective deformation portion (63, 64) and the adjoining mass portion (65, 66).
 10. Apparatus according to claim 8, characterized in that at least the first deformation portion (63) comprises one of said signal generator (67) which extends over at least the entire length of the deformation portion (63).
 11. Device according to claim 10, characterized in that the one of said signal generator (67) extends over the entire length distance between the mass centres (S1, S2) of gravity of the first and second mass portions (65, 66).
 12. Device according to claim 8, characterized in that the first and second deformation portions (63, 64) due to a smaller cross-sectional area have a higher deformability than the first and second mass portions (65, 66).
 13. Device according to claim 8, characterized in that the first and second deformation portions (63, 64) consist of a material of higher elastic deformability than the first and second mass portions (65, 66).
 14. Device according to claim 8, characterized in that the first and second deformation portions (63, 64) have the same deformability and the same cross-section as the first and second mass portions and the borders between the deformation portions and the mass portions are defined by the position of at least one of said signal generators (68, 69, 70). 